Hang up both a "warning; guard dog" sign, AND a "do not tresspass" sign. You'll need both if you don't want to be sued by someone claming to have been bitten.
Actually, even signs won't necessarily protect someone from a lawsuit if someone is bitten. I've read about burglars robbing someone's house, getting bit, sueing the owners and winning. Same as if you slip on someone's icy sidewalk and break a leg, the homeowner is held liable for injury occurring on their land.

Best of luck with Nicky. The advice others have given here is good advice.